Holes are drilled onshore and off-shore for getting access to oil fields and gas fields. These fields are located underground in one of the geological layers.
When drilling a borehole a drilling fluid is used to transport cuttings out of the borehole. With borehole depths of some hundreds of meters up to some kilometers the hydrostatic pressure at the bottom of the borehole could be some hundreds of bars.
Because of these high hydrostatic pressures, the drilling fluid has the tendency to penetrate the geological formations. When entering the formation layer, in which the energy source, such as oil or gas, is located, the drilling fluid could penetrate this layer through which this layer gets clogged and the production of gas or oil is affected. This problem arises especially with low pressure fields.
It is known to adjust the density of the drilling fluid in order to adjust the hydrostatic pressure at the bottom of the borehole. However this hydrostatic pressure variation generally does not correspond to the pressure variation in the specific formation layer. Especially when drilling a borehole in a low pressure field, the maximum pressure of this field could be substantially lower than the hydrostatic pressure of the drilling fluid.